Over fifty years ago it was recognized that alkylbenzene sulfonates (ABS) were quite effective detergents superior to natural soaps in many respects. Because of their lower price, their price stability, and their effectiveness in a wide range of detergent formulations, ABS rapidly displaced soaps in household laundry and dishwashing applications and became the standard surfactants for the detergent industry.
The alkylbenzene sulfonates had substantial branching in the alkyl chain until the early 1960's when it became apparent that these detergents were contributing to the pollution of lakes and streams and forming relatively stable foams. Examination of the problem showed that alkyl chains with a branched structure were not susceptible to rapid biodegradation and the surfactant properties of the detergent thus persisted for long periods of time. This was contrary to the earlier situation when natural soaps were used because the linear alkyl chains in natural soaps underwent rapid biodegradation.
After recognizing the biodegradability of ABS based on alkylation by linear olefins, industry turned its attention to the production of these unbranched olefins and their subsequent use in the production of linear alkylbenzenes. Processes were developed for efficient alkylation of benzene by available feedstocks containing linear olefins, and the production of linear alkylbenzenes (LABs) became another reliable process broadly available to the petroleum and petrochemical industry. It gradually evolved that HF-catalyzed alkylation was particularly effective in LAB production, and an HF-based alkylation process became the industry standard.
At this point the definition of several terms are necessary to adequately understand and appreciate what follows. Alkylation typically is performed using an excess of benzene relative to olefins. The ideal process would afford 100% conversion of olefins using an equimolar proportion of benzene and olefins, but since this is not attained one strives for maximum olefin conversion using a benzene to olefin molar ratio up to about 30. The better the process, the lower will be the benzene:olefin ratio at a high conversion of, say, 98%. The degree of conversion at a constant value of benzene-olefin ratio is a measure of catalytic activity (subject to the caveat that the ratio must not be so high that the degree of conversion is invariant to small changes in this ratio). The degree of conversion may be expressed by the formula, ##EQU1## where V equals percent conversion, C equals moles of olefin consumed, and T equals moles olefin initially present.
However active the catalyst may be, a process based on the catalyst also must be selective. Selectivity is defined as the percentage of total olefin consumed under reaction conditions which appears as monoalkylbenzene and can be expressed by the equation, ##EQU2## where S equals selectivity, M equals moles of monoalkylbenzenes produced, and C equals moles olefin consumed. The better the selectivity, the more desirable the process. An approximate measure of selectivity is given by the equation, ##EQU3## where "total products" includes monoalkylbenzenes, polyalkylbenzenes, and olefin oligomers. At high selectivity (S&gt;85%) the results calculated from the two equations are nearly identical. The latter of the foregoing two equations is routinely used in commercial practice because of the analytical difficulty in distinguishing between oligomers and polyalkylbenzenes.
Finally, the reaction of linear olefins with benzene in principal proceeds according to the equation, EQU C.sub.6 H.sub.6 +R.sub.1 CH=CHR.sub.2 .fwdarw.C.sub.6 H.sub.5 CH(R.sub.1)CH.sub.2 R.sub.2 +C.sub.6 H.sub.5 CH(R.sub.2)CH.sub.2 R.sub.1.
Note that the side chain is branched solely at the benzylic carbon and contains only one branch in the chain. Although strictly speaking this is not a linear alkylbenzene, nonetheless the terminology which has arisen for the process and product in fact includes as linear alkylbenzenes those materials whose alkyl group chemically arises directly from linear olefins and therefore includes alpha-branched olefins. Because alkylation catalysts also may induce the rearrangement of olefins to ultimately give products which are not readily biodegradable (vide supra), for example, .alpha.,.alpha.-disubstituted olefins which subsequently react with benzene to afford an alkylbenzene with branching at other than the benzylic carbon, ##STR1## the degree to which the catalyst effects formation of linear alkylbenzenes is another important catalyst parameter. The degree of linearity can be expressed by the equation, ##EQU4## where D equals degree of linearity, L equals moles of linear monoalkylbenzene produced, and M equals moles of total monoalkylbenzene produced.
Consequently, the ideal process is one where V equals 100, S equals 100, and D equals 100. The minimum requirement is that linearity be at least 90% at a selectivity of at least 85% and at a conversion of at least 99%. These are minimum requirements; that is, if a catalyst fails to meet all of the foregoing requirements simultaneously the catalyst is commercially unacceptable. Moreover, the linearity requirement is assuming added importance and significance in view of the expectation in some areas of minimum standards for linearity in detergents of 92-95% near-term, increasing to 95-98% by about the year 2000. Since the olefinic feedstock used for alkylation generally contains a small percentage of non-linear olefins--a non-linear olefin content of about 2% is common to many processes--the requisite linearity in the detergent alkylate places even more stringent requirements on catalytic performance; the inherent linearity of the alkylation process must increase by the amount of non-linear olefins present in the feedstock. For example, with a feedstock containing 2% non-linear olefins the catalyst must effect alkylation with 92% linearity in order to afford a product with 90% linearity, and with a feedstock containing 4% non-linear olefins the catalyst must effect alkylation with 94% linearity to achieve the same result.
Our solution to the problem of identifying a catalyst for detergent alkylation which satisfies all the aforementioned criteria, and which in particular meets the increasingly stringent requirements of linearity, arose from the observation that the isomerization of linear olefins to non-linear olefins--this is the process ultimately responsible for non-linear detergent alkylate arising from a linear olefin feedstock--is quite sensitive to temperature but relatively insensitive to the particular candidate catalyst for the detergent alkylate process. This result was itself quite surprising, but more importantly it suggested that effecting alkylation at a lower temperature was the key to greater product linearity.
The importance of the observation that temperature is the major factor in olefin isomerization, with the particular catalyst playing only a minor role, cannot be overemphasized, for it permits one to focus solely on methods of reducing the alkylation temperature in order to minimize olefin isomerization and thereby maximize linearity. Since the remaining requisites of a detergent alkylation process can be addressed in other ways, our observation significantly foreshortens the focus on ways to obtain an improved process.
Turning to the object of reducing alkylation temperature, there are two distinct and quite different approaches to achieving this end which we shall refer to as the direct and indirect approaches. The direct approach is to increase catalyst activity. Clearly, as catalyst activity increases--i.e., increasing the rate of reaction of the olefin with benzene at some set of standard reaction conditions--one can lower the alkylation temperature and still attain the requisite conversion and productivity, (the amount of detergent alkylate produced per unit time).
The indirect route of reducing detergent alkylate is to increase catalyst stability. With time, every alkylation catalyst deactivates, and both conversion and productivity decrease to a point where the catalyst must be taken out of service and be regenerated or replaced. The time between regenerations or replacement may be referred to as the lifetime of the catalyst, and the customary solution to a decrease in catalyst stability is to increase operating temperature. The corollary of this is that if an increase in catalyst stability can be effected then the operating temperature may be decreased. To summarize, a consequence of increasing catalyst stability is that one can decrease alkylation temperature without any adverse effects. Since the degree of linearity in linear alkylbenzenes is more highly dependent on the alkylation temperature than on the nature of the alkylation catalyst any process change which permits a lower alkylation temperature leads to an increase in linearity of LABs. It then follows that a consequence of increasing catalyst stability is to produce LABs with a higher linearity because of the reduction in alkylation temperature made possible by the increased catalyst stability.
What we have found is that silica-aluminas having an ultra-low sodium content are considerably more stable than silica-aluminas with a more "normal" sodium content. Typically, silica-aluminas have a sodium content of at least 0.1 weight percent with sodium typically being in the range of 0.1-0.3 weight percent or more. The ultra-low silica-aluminas of this invention have a sodium content of less than 0.1 weight percent. The stability increase attending an ultra-low sodium content is manifested not only by the usual silica-aluminas, but also is manifested by fluorided silica-aluminas.
The use of silica-aluminas as a support for various metals in the alkylation of aromatics with olefins is reasonably well known. For example, U.S. Pat. No. 3,169,999 teaches a catalyst consisting essentially of small amounts of nickel and chromia on a silica-alumina support, and U.S. Pat. No. 3,201,487 teaches 25-50 weight percent chromia on a silica-alumina support, both for alkylation of aromatics by olefins. Crystalline alumina-silicates as catalysts in detergent alkylation has been described in U.S. Pat. Nos. 4,301,317 and 4,301,316. U.S. Pat. No. 4,358,628 claims an alkylation process with an olefin using as a catalyst tungsten oxide supported on a porous silica-alumina support containing 70-90% silica prepared in a very particular way.
More relevant is European Patent Application 0160145 which teaches as a catalyst in detergent alkylation an amorphous silica-alumina having specified channels or networks of pores and with at least 10% of the cationic sites occupied by ions other than alkali or alkaline earth metals. An even more relevant teaching appears in U.S. Pat. No. 4,870,222 where the patentees teach a process for the production of a monoalkylated aromatic compound in which an aromatic is first alkylated, the product mixture is separated, and the polyalkylated material thereafter is transalkylated, and where the most preferred catalyst for alkylation is an amorphous, silica-alumina material, particularly a cogelled silica-alumina prepared as spheroidal particles by the oil drop method.